Improvements in alloys may enable disc rotors in gas turbine engines, such as those in the high pressure (HP) compressor and turbine, to operate at higher compressor outlet temperatures and faster shaft speeds. These properties may facilitate high climb rates that are increasingly required by commercial airlines to move aircraft more quickly to altitude, to reduce fuel burn, and to get the aircraft away from busy air spaces around airports.
The above mentioned operating conditions may give rise to fatigue cycles with long dwell periods at elevated temperatures, in which oxidation and time dependent deformation significantly influence the resistance to low cycle fatigue. As a result, it would be desirable to improve the resistance of alloys to dwell fatigue or time dependent crack growth and surface environmental damage, and to increase proof strength, without compromising their other mechanical and physical properties or increasing their density and cost.
Current alloys cannot provide the balance of properties needed for such operating conditions. Many are claimed to show excellent creep resistance, high temperature yield strength and damage tolerance under dwell cycles at temperatures in the range of 600° C. to 760° C. as well as microstructural stability. However, their resistance to environmental damage, particularly hot corrosion resistance is not optimised. Many prior alloys show high density (close to or exceeding 8.5 g·cm−3) and are expensive, given the high levels of tantalum.
Current nickel base alloys have compromised resistance to surface environmental degradation (oxidation and type II hot corrosion) in order to achieve improved high temperature strength and resistance to creep strain accumulation, and in order to achieve stable bulk material microstructures (to prevent the precipitation of detrimental topologically close-packed phases). Disc rotors in the High Pressure (HP) section are commonly exposed to temperatures above 650° C., and in future engine designs will be exposed to temperatures above 730° C. As disc temperatures continue to increase, hot corrosion and oxidation damage will begin to limit disc life. Without suitable alloys, environmental protection will need to be applied to such discs, which may be undesirable and technically very difficult.